cleanup: Always use single quotes

We have made good progress with transitioning to the new style,
to the point where we can complete it with a final push.

Start with changing the remaining places that still use double
quotes.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2866>

js/ui/overview.js

@@ -73,7 +73,7 @@ class OverviewActor extends St.BoxLayout {
             name: 'overview',
             /* Translators: This is the main view to select
                 activities. See also note for "Activities" string. */
-            accessible_name: _("Overview"),
+            accessible_name: _('Overview'),
             vertical: true,
         });